The subject matter described herein relates generally to power conversion within electric power systems, and more specifically, to low-inductance, three-level, neutral point clamped (NPC) power converters for electric power generation assets.
Generally, a wind turbine includes a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. At least some of the known wind turbines are physically nested together in a common geographical region to form a wind turbine farm. Variable speed operation of the wind turbine facilitates enhanced capture of energy when compared to a constant speed operation of the wind turbine. However, variable speed operation of the wind turbine produces electric power having varying voltage and/or frequency. More specifically, the frequency of the electric power generated by the variable speed wind turbine is proportional to the speed of rotation of the rotor. A power converter may be coupled between the wind turbine's electric generator and an electric utility grid. The power converter receives electric power from the wind turbine generator and transmits electricity having a fixed voltage and frequency for further transmission to the utility grid via a transformer. The transformer may be coupled to a plurality of power converters associated with the wind turbine farm.
Many known power converters include a plurality of power conversion devices, i.e., power modules that include semiconductor devices such as insulated gate bipolar transistors (IGBTs). The IGBTs, other electronic devices, and the associated electrically conductive connections that form the power modules have known inductances and the associated impedances. The impedances are proportional to the frequencies of the signals transmitted therethrough. As such, transmitting electric current through such power modules at high switching frequencies induces electromagnetic fields that may induce unwanted voltages and currents, thereby increasing switching losses and voltage overshoots. Significantly, some diode devices will experience a reverse recovery that induces a voltage and current spike that is transmitted through briefly-formed commutation loops. The deleterious effects of such commutation loops are proportional to the associated inductance of the loops, and such effects may include high voltage stresses on the semiconductor devices. Moreover, such unwanted voltages and currents may include harmonics affecting the power quality of the electric power transmitted from the power converters. As the switching losses increase and demands for more robust power converters escalate, the size, weight, and cost of the power modules, and, therefore, the power converters, increase proportionally to compensate.
Three-level bridge configurations that generate three voltages, i.e., a positive voltage, a neutral voltage, and a negative voltage, facilitate faster power converter switching speeds, i.e., rates of voltage transitions measured at the output terminals of the converter, than other bridge configurations, thereby facilitating improved power quality. However, configuring such three-level bridges, with the associated heat sinks, as well as the power conversion components and the conductors therebetween, significantly increases the challenges associated with maintaining the inductances of the power modules low enough to reduce the negative impact of higher switching speeds of the power converters. Limiting the switching speeds of the power converters due to the inherent inductances of the modules therein limits the performance advantages associated with the higher switching speeds, and facilitates increased size, weight, and cost of power converters.